The present invention relates to a process for production of abrasive foams based on a melamine-formaldehyde condensate, and also to the products produced by this process. The process uses heating to foam a preferably aqueous solution or dispersion which comprises a melamine-formaldehyde precondensate, an inorganic component in the form of nanoparticles, and also, if appropriate, further components, such as an emulsifier, a blowing agent, a hardener, and also further additives. The foam can then be hardened via crosslinking of the precondensate and further processed to give various downstream products.
Inorganic nanoparticles are increasingly important in many sectors of industrial production. The particular properties of nanoparticle materials give them interesting possible uses ranging from chip technology to rubber production to medicine and cosmetics. Nanoparticles generally have physical and chemical properties markedly different from those of their coarse-grain analogs. This leads to particular possible applications, since it is possible to make a very high, and also chemically variable, surface area available for a small particle volume. For example, nanoparticles can be used for very effective scattering of energy arising within a material, and this has led by way of example to increased elasticity of tires and reduced rolling resistance in the rubber industry sector. However, inorganic nanoparticles are also useful in the household and cleaning sectors.
Various processes have been previously disclosed for production of foams and also of abrasive foams. For example, EP-A 017 672 described as early as 1979 elastic foams based on melamine-formaldehyde condensates, and also a process for their production. In this process, a highly concentrated solution or dispersion of a melamine-formaldehyde precondensate comprising a blowing agent is foamed and the foam is hardened. The foaming can take place via heating to a temperature above the boiling point of the blowing agent and can be carried out in such a way as in essence to delay any steep viscosity rise until the foaming procedure has been ended. The heating is preferably achieved with hot air, but can also use steam, high-frequency irradiation, or utilize heat of reaction. In EP-A 017 672 it is also possible to add pulverulent inorganic components as fillers or flame retardants to the foams.
The document NL-A 102 4682 describes open-cell rigid foams, for example based on melamine-formaldehyde, to which it is also possible to add inorganic additions prior to or after the foaming process. However, the materials involved here are not flexible melamine foams but rigid foams. These have physical properties which are different from those of the inventive flexible foams. The additions, such as sand, are not selected in NL-A 102 4682 with regard to increased abrasiveness of the foam.
The document WO 2006/003421 describes flexible polyurethane foams which can comprise nanoparticles based on phyllosilicates (e.g. cloisites).
WO 2005/100497 discloses plastics comprising pores and composed of a porous polymer, in which the pores have to some extent been filled with abrasive particles composed of metal oxides (e.g. composed of aluminum oxides, silicon oxides, titanium oxides, cerium oxides, zirconium oxides, germanium oxides, or magnesium oxides). These are used for the polishing of semiconductor elements, such as wafers. The polymers are based on polyurethanes and on polyolefins. In the examples section, a process is presented for the opening of pores during foaming via application of suction.
CA-A 2 293 906 describes foams for household applications, which are produced via impregnation of the upper layer of a roughened foam product with a coating which comprises abrasive additions, e.g. calcium carbonate, hollow glass microbeads, quartz, silicon carbide, or aluminum oxide.
JP-A 2001/287152 describes polishing pads for the surface treatment of wafers, which are obtained via impregnation of polyurethane foams with abrasive impregnation materials (e.g. silicon dioxide or cerium oxide) and a subsequent pressure process.
EP-A 1 683 831 discloses the addition of up to 5% by weight of inorganic nanoparticles to the components of a polyurethane, and subsequent foaming to give a polyurethane foam. Here, the nanoparticles are used as nucleating agents for foam-structure control.
U.S. Pat. No. 3,653,859 describes the impregnation of a foam with a dispersion which comprises abrasive additions.
US 2003/207954 describes polyurethane foams which can comprise abrasive additions, e.g. minerals, such as feldspar.
WO 1999/24223 also describes foams which have abrasive particles, e.g. composed of aluminum oxide or quartz.
JP-A 2003/175466 describes the impregnation of Basotect® foam (producer: BASF) with abrasive particles and with an elastomeric binder.
EP-A 1 742 980 describes the covalent linkage of inorganic nanoparticles to the cell-wall structure of Basotect, in order to achieve dirt-repellant action.
The German utility model DE 298 08 991 proposes, for the polishing of plastics surfaces, use of a polishing paste in which the abrasives have been dispersed in a carrier substance in which the viscosity falls under the conditions of the polishing procedure.
DE-A 31 02 567 discloses a paste for the polishing of metal surfaces, in which uniformly distributed portions of powder have been introduced into the melt of the paste, their hardness being between the hardness of the grinding particles and the hardness of the melt. Powders mentioned here are metal particles, such as copper powders, aluminum powders, zinc powders, and brass powders.
Familiar abrasives and polishing pastes often use fine-grain, high-hardness grinding particles, but these can, during the polishing procedure, lead to tiny scratches on the surface to be polished. In many sectors of technology (e.g. production of electronic components), but also in the household sector, these scratches on surfaces are undesirable. There are also foams used which do not have high and long-lasting elasticity and which therefore can be used only for a very restricted period, and this is undesirable for environmental reasons.
There are various ways of combining a foam with abrasive particles to provide an abrasive foam:                impregnation of the foam with a dispersion of the particles (with addition of binder or else with covalent linkage),        addition of abrasive particles to a polishing paste        addition of abrasive particles to the precondensate prior to the foaming process,        addition of abrasive particles to the starting materials in preparation of the precondensate and foaming in the subsequent step,        a combination of the processes listed above.        